The present invention generally relates to wireless communication services, and particularly relates to tracking signal impairment correlations of received communication signals from multiple base stations.
RAKE receivers are well known in the communication arts and find widespread use in Code Division Multiple Access (CDMA) systems, such as in IS-95, IS-2000 (cdma2000), and Wideband CDMA (WCDMA) wireless communication networks. The name derives from the rake-like appearance of such receivers, wherein multiple, parallel receiver fingers are used to receive multiple signal images in a received multipath signal. By coherently combining the finger outputs in a RAKE combiner, the conventional RAKE receiver can use multipath reception to improve the Signal-to-Noise Ratio (SNR) of the received multipath signal.
However, as is known to those skilled in the art, the conventional RAKE receiver is optimal only in certain limited circumstances. For example, the presence of self-interference and multi-user access interference both degrade the performance of a conventional RAKE receiver. To that end, the assignee of the instant application has made application for one or more patents relating to the use of a “generalized” RAKE (G-Rake) receiver architecture, wherein receiver performance is improved by increasing the sophistication of combining weight generation.
In the G-Rake architecture, the combining weight calculations consider correlations of one or more signal impairments across RAKE fingers. For example, a G-Rake receiver may track noise correlations across those fingers. G-Rake receivers also may include a comparatively larger number of fingers such that extra fingers may be positioned off of the signal path delays. Indeed, a G-Rake receiver can gain performance improvements by shifting these extra fingers to maximize the SNR of the received signal. Correlations of signal impairments can also be used in SNR estimating often referred to as signal to interference ratio (SIR) estimation. SIR estimation is used in power control, rate control, and in monitoring link quality. The term “RAKE” is used herein generally to refer to RAKE and G-Rake processing.
By using its knowledge of how selected signal impairments are correlated across fingers, the G-Rake receiver can compensate the finger combining weights such that receiver performance is improved. Of course, the need to determine signal impairment correlations with sufficient accuracy and rapidity stands as a primary challenge associated with implementation of the G-Rake receiver.
Parent application Ser. No. 10/800,167, entitled METHOD AND APPARATUS FOR PARAMETER ESTIMATION IN A GENERALIZED RAKE RECEIVER, filed Mar. 12, 2004 and incorporated herein by reference in its entirety, describes a method and apparatus to estimate signal impairment correlations for one or more received signals of interest using a model-based technique. According to this technique, the model is adapted responsive to recurring measurements of signal impairment correlations that can be made on a frequent basis, e.g., every timeslot of a Wideband CDMA (WCDMA) frame, thereby dynamically tracking even rapidly changing signal impairment correlations. A method of determining received signal impairment correlations for use in generating G-Rake combining weights and/or SIR estimates comprises providing a model of received signal impairment correlations comprising one or more impairment terms scaled by corresponding model fitting parameters, and adapting each of the model fitting parameters responsive to recurring measurements of the received signal impairment correlations such that the model of received signal impairment correlations dynamically tracks changing reception conditions.
In particular, in one embodiment of this method the model of received signal impairment correlations is Ru=αRI+βRn where RI is an interference correlation matrix, and Rn is a thermal noise correlation matrix arising from the autocorrelation properties of receiver filtering. The method comprises the following steps, performed at each of a number of repeating time intervals:                a. Measure impairment correlations for the received signal in the current slot, i.e., determine a rough estimate of impairment correlations expressed as matrix {circumflex over (R)}(slot);        b. Calculate per-slot model terms RI(slot) and Rn(slot) based on estimated channel coefficients;        c. Determine instantaneous model fitting parameters αinst and βinst for the slot based on performing a Least Squares fit of,{circumflex over (R)}u(slot)≈αinstRI(slot)+βinstRn(slot);        d. Update the model fitting parameters α and β based on the instantaneous fitting parameters, e.g., update filtered values of α and β using αinst and βinst; and        e. Calculate the modeled impairment correlations {tilde over (R)}u(slot) to be used in generating RAKE combining weights and an SIR estimate for the current slot as,{tilde over (R)}u(slot)=αRI(slot)+βn(slot).        
The parent application additionally describes a model of received signal impairment correlations in which interference from another base station is considered, i.e., {circumflex over (R)}u=αRI+βRn+γR0 where RI is the same-cell or own-cell interference correlation matrix and R0 corresponds to other-cell interference. The other-cell interference may be estimated in a number of ways, such as modeling it as white noise that has passed through the transmit pulse shaping filter.
In one embodiment disclosed in the parent application, during soft handoff, a set of receiver RAKE fingers is allocated to the signal from a first base station, and another set of RAKE fingers is allocated to the signal from a second base station. When computing combining weights for the first base station signal, the receiver treats the second base station signal as other-cell interference. When computing weights for the second base station signal, it treats the first base station signal as other-cell interference. However, in each case, the method measures received signal impairment correlations considering only the pilot signal from the respective own-cell base station.